Electromagnets are capable of inducing electric fields in most biological tissues. Transcranial magnetic stimulation (TMS) is widely used as a research tool to study aspects of the human brain, including motor function, vision, language, and brain disorders. Additionally, therapeutic uses of magnetic stimulation devices, particularly in psychiatry, currently are being investigated.
Magnetic stimulation of biological tissue may be accomplished by passing a brief, high-current electric pulse through a coil of electrically conductive material, such as a wire positioned adjacent tissue to be stimulated. A magnetic field is produced by the electric pulse with lines of flux passing perpendicularly to the plane of the magnetic coil. This magnetic field, in turn, can induce an electric field in a conductive medium. An animal brain is a conductive medium and in TMS, the induced electric field stimulates the neurons of the brain. However, an electromagnetic coil may be placed over other parts of the body to stimulate other electrically conductive tissues, such as muscle.
Functional magnetic coils may be produced in a variety of shapes including circles, figure-8's, squares, petals, spirals, and “slinky” coils. See, e.g. Caldwell, J., Optimizing Magnetic Stimulator Design, Magnetic Motor Stimulation: Basic Principles and Clinical Experience, 1991, 238-48 (ed. Levy, W. J., et al.); Zimmermann, K. P., and Simpson, R. K., Electroencephal. Clin. Neurophysiol., 101:145-52 (1996); U.S. Pat. No. 6,066,084 (Edrich et al.). The coils may include features other than a coil of a transducing material. For example, U.S. Pat. No. 6,086,525 (Davey et al.) and WO 98/06342 (Epstein et al.) disclose magnetic stimulators made from coil windings around a core of ferromagnetic material, preferably vanadium permendur. However, such coils can be quite heavy and expensive to manufacture.
TMS using known coils has been shown to be able to stimulate the regions of the brain close to the surface of the skull, but magnetic fields produced by these known coils generally do not penetrate deeply into the brain, unless the intensity of the magnetic field is greatly increased. However, increasing the strength or intensity of the magnetic field carries a risk of causing physiological damage and seizures.
The deep regions of the brain include the nucleus accumbens, a portion of the brain that plays a major role in rewarding circuits and is known to be activated in response to doses of cocaine. Additionally, neuronal fibers connecting the medial, prefrontal, or cingulate cortex with the nucleus accumbens have a role in reward and motivation, and activation of the nucleus accumbens also may cause hedonic effects.
Known coils used for TMS (e.g., a figure eight coil) affect the cortical regions of the brain, primarily the cortical region under the center of the coil. However, the intensities of the electric fields produced by these known coils decrease very rapidly with increasing distance from the coil. Therefore, stimulating deep regions of the brain using known coils would require either invading the skull (and often the brain) with the coil, or using a high intensity electric field. Invasive techniques often cause the subject or patient to experience pain or discomfort, and would usually be avoided by the patient. High intensity electric fields may cause epileptic seizures or other neurological problems. Moreover, high intensity electric fields may cause generalized effects throughout a subject's brain, rather than stimulating a specific deep region of the brain, and may cause other harmful side effects. Additionally, the maximum field intensity can be limited by known coil designs.
Therefore, a need exists for a magnetic coil capable of stimulating the deep regions of the brain when placed outside the skull during non-invasive TMS.